Refrigeration is the use of mechanical or heat-activated machinery for cooling purposes. Refrigeration is commonly accomplished in a reverse Carnot cycle, by using as refrigerant a fluid that evaporates and condenses at suitable pressures and temperatures to enable practical equipment to be manufactured. In a vapor compression refrigeration cycle, the vapor is typically compressed, then condensed by chilling with air or water, then expanded to a low pressure and correspondingly low temperature through an expansion valve. Subsequent evaporation of the refrigerant provides the cooling action. In an absorption refrigeration cycle, cooling is also achieved by expansion of a high-pressure vapor into a low-pressure region. The resulting low-pressure vapor is absorbed into water, then separated from the water at high pressure in a stripper.
Many fluids that can serve as refrigerants under appropriate conditions are known. Refrigerants are generally grouped into three classes, depending on their toxicity and flammability. Group 3 refrigerants are highly toxic or flammable, and are therefore used only in special circumstances, such as where the refrigerant is available on-site as a process or product chemical, and the existing hazard is not exacerbated by the use. Such refrigerants include hydrocarbons such as methane, propane and butane. Group 2 refrigerants are slightly toxic or flammable, and include ammonia, which is still used widely, as well as sulfur dioxide. Group 1 refrigerants are non-toxic and non-flammable, and are, therefore, the most widely used over a broad spectrum of refrigeration needs. Mosts of the Group 1 refrigerants are halogenated hydrocarbons, containing one or more chlorine, fluorine or bromine atoms in their structures. For example, industrial refrigerators use vast quantities of CFC-12 and other chlorofluorocarbons (CFCs), which, although they are non-toxic and non-flammable, are now recognized to have a disastrous environmental impact.
Refrigeration can be carried out either as a closed-cycle or open-cycle process. Open-cycle operation is mostly used in the chemical process industry, where advantage is taken of the presence in the chemical process of a product that can also serve as refrigerant. For example, natural gas liquids removed by cooling and compressing raw natural gas may be expanded in a refrigeration cycle to further lower the temperature of the raw gas, thereby recovering more of the heavier hydrocarbons. Ammonia synthesis plants use the product stream to refrigerate ammonia storage tanks.
For most other industrial purposes, closed-cycle refrigerators are used. The refrigerant is contained in an essentially closed loop, where it cycles round from high-pressure vapor to high-pressure liquid to low-pressure liquid to low-pressure vapor. The low-pressure, evaporating portion of the system may be at atmospheric pressure or may be below atmospheric pressure, depending on the thermodynamic properties of the refrigerant and the cooling temperature. For practical reasons, refrigeration systems using CFC refrigerants are frequently operated with the evaporating pressure as low as 2-5 psia.
Because a large portion of the refrigeration system is at sub-atmospheric pressure, air leaks into the system on the low pressure side. Air leaks are almost unavoidable in large industrial refrigerators; thus air contaminated with refrigerant vapor must be periodically purged from the system. In conventional purge systems, a gas stream, containing refrigerant and air, is withdrawn from the high-pressure side of the cycle. To reduce the refrigerant loss, the stream is maintained at the high purge pressure and then cooled, typically down to as low as -50.degree. F. or below. The low-temperature refrigerant can conveniently be used to effect the cooling. Under these conditions, the bulk of the refrigerant contained in the stream is condensed and passed back to the refrigerator. The remainder is vented to the atmosphere. The frequency and thoroughness with which the purging operation is carried out is dictated by energy and economic considerations. If the air content within the loop is allowed to build up over a prolonged period, the partial pressure of the air in the system may become substantial. As a result, the total compressor pressure required to maintain the refrigerant partial pressure at an adequate level becomes higher and higher, with a corresponding increase in energy consumption and costs.
The air content of the refrigerator can be kept at a constant low level by continuous purging. Cooling the purge gas typically enables as much as 90% or more of the refrigerant to be recovered from the purge stream by condensation. Nevertheless the air that is vented to the environment may contain as much as 15% refrigerant. Running the purge-gas treatment condenser at pressure and temperature conditions where essentially no refrigerant is lost imposes an excessively heavy load on the condenser, consumes excessive energy, and becomes impractical economically. The need to drastically control or eliminate CFC emissions to the atmosphere has been recognized throughout the world and is the subject of increasingly stringent regulatory laws. CFC refrigerants, besides their environmental unacceptability, are becoming increasingly expensive. Refrigerator discharges represent a serious environmental problem and waste of resources. A 10 scfm condenser vent stream containing 5% or more CFC is typical of many that are found throughout the food and pharmaceutical industries, for example. Such a discharge corresponds to a CFC loss of 0.16 lb/min, or approximately 80,000 lb/yr. When multiplied by the many hundred industrial refrigeration plants in use nationwide, this rate of loss represents a large source of CFC pollution and waste resources. Thus there is an urgent need to improve refrigeration technology to drastically reduce or eliminate CFC discharges. Similar, if less critical, concerns apply to other refrigerants. Because of the adverse effect on the operation of the refrigeration cycle, there is also a need for improved methods of keeping the air content of the cycle as low as possible.
Attempts have been made to monitor and/or treat purge streams from refrigeration operations by various means besides condensation. For example, U.S. Pat. No. 4,485,289 to Lofland describes a distillation process for recovering CFCs from refrigerator purge streams. U.S. Pat. No. 4,531,375 to Zinsmeyer describes a refrigeration system including means for monitoring a refrigerator purge system and correcting excess discharge of purge gases. U.S. Pat. No. 4,484,453 to Niess describes a method for controlling non-condensable gases at a predetermined concentration in an ammonia refrigerator by sensing the temperature at which the ammonia condenses.
Separation of gas or vapor mixtures by means of permselective membranes has been known to be possible for many years, and membrane-based gas separation systems are emerging to challenge conventional separations technology in a number of areas. That membranes have the potential to separate organic or inorganic vapors from air is known. For example, U.S. Pat. No. 4,553,983, commonly owned with the present invention, describes a process for separating airstreams containing low concentrations of organic vapor (2% or less) from air, using highly organic-selective membranes. U.S. Pat. No. 3,903,694 to Aine describes a concentration driven membrane process for recycling unburnt hydrocarbons in an engine exhaust. U.S. Pat. No. 2,617,493 to Jones describes separation of nitrogen from concentrated hydrocarbon feedstreams. Pending patent application Ser. No. 327,860, now U.S. Pat. No. 4,906,256, commonly owned with the present invention, describes a membrane separation process for treating air or other gas streams containing fluorinated hydrocarbons, such as CFCs.